Polyimide foams

ABSTRACT

Copolyimide foams derived from a diester of 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic acid, an aromatic diamine, and a heterocyclic diamine. A molar concentration of the heterocyclic diamine approaching but not exceeding 0.42 is employed. This results in a flexible foam with a homogeneous cellular structure and a reduced compression set loss.

The invention described herein was made in the performance of work underNASA Contract No. NAS9-15484 and is subject to the provisions of Section305 of the National Aeronautics and Space Act of 1958 (72 Stat. 435; 42USC 2457).

This application is a division of application Ser. No. 252,902 filedApr. 10, 1981, now U.S. Pat. No. 4,332,656.

In one aspect the present invention relates to novel, improved,polyimide foams which are flexible and have a homogeneous cellularstructure and a reduced compression set loss.

The present invention also relates to precursors of foams with thecharacteristics identified in the preceding paragraph, to methods formaking such precursors, and to methods for converting the precursors tothe foams.

U.S. Pat. No. Re. 30,213 issued Feb. 12, 1980, to John Gagliani et alfor Method of Making Foamed Copolyimides and Products Obtained Therefromand copending U.S. patent application Ser. No. 935,378 filed Aug. 21,1978, by John Gagliani for Polyimides (now U.S. Pat. No. 4,241,193issued Dec. 23, 1980) disclose hydrolytically stable copolyimide foamswhich are fire resistant and give off essentially no smoke or toxicfumes when they are heated to degradation temperatures. Consequently,those foams are useful in aircraft cabins, space vehicles, and land andsea transport and in a variety of other applications where human life orequipment might be endangered by the overheating of conventional, moreflammable, smoke-emitting materials.

The polyimide foams described in the foregoing patent and patentapplication are derived from precursors which contain an alkyl diesterof 3,3',4,4'-benzophenonetetracarboxylic acid, an aromatic diamine, anda heterocyclic diamine.

The general model for the chemical reactions which are effected inconverting the precursor to a polyimide of the character discussed aboveare shown below. The actual reactions are typically much more complexdepending upon the number of diamines in the precursor: ##STR1##

We have now discovered that the compression set properties of such foamscan be improved if the molar ratio of heterocyclic diamine totetracarboxylic acid diester(s) in the precursor is maintained in therange of about 0.4-0.42:1.0.

Compression set is a measure of the extent to which a foam will take ona permanent set or deformation after having been compressed to a statedfraction of its original thickness for a prolonged period of time. Thisis important in seating applications, for example; materials which aresusceptible to compression set reach the point where tactile comfortbecomes unacceptable much sooner than those having good compression setproperties.

Generally speaking, then, our novel polyimide foams disclosed herein areprepared from precursors which are solutions of a lower alkyl ester of3,3',4,4'-benzophenonetetracarboxylic acid or a mixture of such esters,an aromatic diamine which is free of aliphatic moieties, and aheterocyclic diamine. The imide-forming functionalities (the amino andcarboxylic moieties) are preferably present in substantially equimolaramounts.

Exemplary of the aromatic and heterocyclic diamines that can be employedare:

2,6-diaminopyridine

3,5-diaminopyridine

3,3'-diaminodiphenyl sulfone

4,4'-diaminodiphenyl sulfone

4,4'-diaminodiphenyl sulfide

3,3'-diaminodiphenyl ether

4,4'-diaminodiphenyl ether

meta-phenylene diamine

para-phenylene diamine

p,p'-methylene dianiline

2,6-diamino toluene

2,4-diamino toluene

The precursors of our polyimide foams are essentially monomeric, solidstate solutions of the selected ester (or esters) and diamines.

They are prepared by first reacting3,3',4,4'-benzophenonetetracarboxylic acid or, preferably, a dianhydrideof that acid with an esterification agent to form an alkyl diester.Exemplary esterification agents are methyl, ethyl, propyl, and isopropylalcohols. Methanol is in many cases preferred because of its widespreadavailability, low cost, and other attributes; because its usefacilitates conversion of the precursor to a polyimide foam; and becausethe foams made from the methyl esters tend to be more flexible,resilient, and compression set resistant. Ethanol is also a preferredesterification agent.

The esterification reaction is followed by the addition of the diamines,which are dissolved in the reaction mixture. The temperature is keptbelow the influx temperature of the esterification agent duringdissolution of the diamines and low enough to avoid polymerization ofthe diamines and ester(s).

A surfactant can be added to the mixture thus formed to increase thefatigue resistance by increasing the bubble stability of the foam andthe uniformity of the cellular structure. One preferred surfactant isZonyl FSB, a nonionic, fluorinated, polyalkylene copolymer manufacturedby E. I. DuPont de Nemours and Company. We have employed from 0.00625 to0.05 percent of this surfactant based on the weight of the ester anddiamine constituents. In systems containing 2,6-diamino pyridine andp,p-methylene dianiline along with a3,3',4,4'-benzophenonetetracarboxylic acid methyl or ethyl ester, aconcentration of ca. 0.0125 percent proved to be optimum.

The material existing after dissolution of the diamines and the additionof any additives is transformed into an amorphous powder capable ofbeing converted into a flexible, resilient, polyimide foam. Although notessential, it is preferred that spray drying be employed for thispurpose because the liquid resin can thereby be economically transformedon a continuous basis and in one step into a dry powder. Also, spraydrying allows for modification of the precursor in ways which can beused to vary the properties of the final product.

One suitable, state-of-the-art, spray drying process is described incopending application Ser. No. 186,670 filed Sept. 12, 1980, which ishereby incorporated herein by reference. In that process the materialexisting after the dissolution of the diamines and the addition of anyadditives is diluted with about 20 parts of an alkyl alcohol per 100parts of resin and atomized, preferably with a rotary atomizer operatedat a speed in the range of 32,000 to 35,000 rpm. The atomized dropletsare dried in a chamber having an inlet and an outlet for a heated gas.The inlet temperature of the gas is maintained in the range of 100°-110°C., and the outlet temperature of the gas is limited to a maximum of 80°C.

The amphorous, powdered, resinoid precursor existing at the end of thespray drying step can be converted to a monolithic, polyimide foam ofthe character discussed above by various techniques includingdielectric, thermal, and microwave heating. The latter, alone or with athermal post-cure, is preferred because of the speed with which the foamcan be generated and cured; because the foam is homogeneously heated;and because handling of the fragile, uncured foam can be avoided.

Microwave techniques and equipment that can be used to foam and cure theprecursor are disclosed in copending application Ser. No. 186,629 filedSept. 12, 1980, which is also intended to be incorporated herein byreference. They can be used to convert the precursor to a foam by thefree rise technique; by an open mold, constrained rise technique inwhich spacers are employed to limit the rise of the foam; or in a closedmold.

Foaming-curing parameters that have proven satisfactory in convertingrepresentative precursors of the character described herein to flexible,resilient foams are two to 55 minutes exposure to high frequencyradiation in an oven operating at a frequency of 915 to 2450 mHz and at3.75 to 15 kW power and a power output to precursor unit weight ratio of0.6 to 1 kW/kg.

The thermal postcure of the polyimide, if employed, is accomplished byheating the product of the microwave heating step in a circulating airoven at a temperature of 500° to 550° F. for 30 to 200 minutes tocomplete the curing of the polyimide.

Preferably, the dry precursor is preheated before it is exposed tomicrowave radiation. This contributes to higher foam rise, a morehomogeneous cellular structure, and a reduced scrap rate. Preheattemperatures of 121.1° to 148.9° C. (250° to 300° F.) for 2-6 minuteshave been successfully employed.

Steady application of the microwave energy is not required, and pulsedor cyclic exposure of the precursor to the microwave energy may evenproduce superior results. Typically, the duration of the microwaveenergy pulses and the intervals therebetween will be on the order of 60and 20 seconds, respectively.

Also, conductive fillers can often advantageously be incorporated in theprecursor to promote its conversion to a polyimide by generatingadditional thermal energy. From 5 to 20 weight percent of activatedcarbon or graphite can be employed for that purpose.

Another technique that can be used to advantage in making polyimidefoams by the practice of the present invention is thermal heating of themicrowave cavity. Temperatures in the range of 121.1° to 232.2° C. (250°to 450° F.) are employed.

Similarly, an optimum product can in many, if not most, cases beobtained by heating the substrate or mold on or in which the precursoris foamed to a temperature of 121.1° to 148.9° C. (250° to 300° F.)before the precursor is exposed to microwave energy.

Also, quality may in many cases be optimized by employing differentmicrowave energy power levels to foam the precursor and to subsequentlycure the foam. For example in making foams from precursors of thecharacter described above in existing equipment, a power level of 10 kWfor foaming followed by one of 15 kW to complete the cure of thepolyimide has been selected as optimum.

The depth and loading of the particulate precursor on the substrate areimportant to the development of a stable, homogeneous structure in thefoam to which the precursor is converted. The powder should be at least0.5 cm deep. Powder loadings as low as 1.6 kg/m² have been successfullyemployed. However, loadings in the range of 4.3 to 15 kg/m² arepreferred; and a loading of ca. 7.7 kg/m² is thought to be optimum.

Also, it has been found with free rise foaming techniques that optimumyields are obtained from square shaped beds rather than those ofrectangular configuration.

From the foregoing it will be apparent to the reader that the primaryobject of our invention resides in the provision of novel, improved,polyimide foams.

A related, and also important, but more specific object of our inventionresides in the provision of polyimide foams with improved compressionset properties.

Still other important, related objects of our invention reside in theprovision of precursors for the novel polyimide foams described in thepreceding objects and in the provision of methods for preparing thoseprecursors and for converting them to polyimides.

Certain important objects of the present invention have been identifiedabove. Other important objects and advantages and additional novelfeatures of the invention will be apparent to those skilled in therelevant arts from the foregoing general description of that invention;from the appended claims; and from the ensuing, detailed discussion anddescription of the invention.

In one exemplary study demonstrating the advantages of our invention,3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA) (322.23 g,1.0 mole) was added to 240 ml of methyl alcohol and 24 ml of water in aone-liter, three-neck flask equipped with a thermometer, a mechanicalstirrer, and a reflux condenser. After addition, the mixture wasrefluxed until clear. The mixture was then refluxed for an additional 60minutes to ensure complete reaction of the BTDA to its half (or di-)ester.

The contents of the flask were then cooled to 40°-50° C. (104°-122° F.).

2,6-Diaminopyridine (2,6 DAP) (43.7 g, 0.4 mole) and p,p'-methylenedianiline (MDA) (119.0 g, 0.6 mole) were added to the half estersolution, and the resulting mixture was heated to 60°-65° C. (140°-149°F.) with agitation for five minutes.

At this stage the resulting liquid precursor was diluted with methylalcohol in a ratio of 20 parts of alcohol per 100 parts of resin.

FSB surfactant was also mixed with the liquid resin at this stage invarying concentrations. Thereafter, a Niro Mobile spray dryer was heatedto an inlet temperature of 100° C. (212° F.) and an outlet temperatureof 70° C. (158° F.) as discussed in copending application Ser. No.186,670. The liquid resin was then fed into the dryer with the feedbeing manually adjusted throughout the operation to keep the dryeroutlet temperature in the range of 69°-71° C. (156°-160° F.).

In each instance this produced a powder which was collected, sievedthrough a Tyler 48 mesh (297 microns) sieve, and rolled for 30 minutesin a round plastic bottle.

Each powder this produced was, essentially, a particulate, solid statesolution of unreacted diamines, 3,3',4,4'-benzophenonetetracarboxylicacid diester, and FSB surfactant.

A foam was produced from each of the powdered precursors using a GerlingMoore Batch Cavity Model 4115 microwave oven operating at a frequency of2450 mHz and a power of 5 kW.

The precursor was spread on a Teflon coated glass cloth substrate andplaced in the microwave cavity at room temperature. After two to twelveminutes of exposure to the microwave field, depending upon theparticular test being conducted, the powder expanded into a homogeneous,cellular foam block. This block was thermally cured into a flexible andresilient foam by heating it at 260° C. (500° F.) for two hours.

The compression set (and other) properties of the foams were thenidentified and compared to the corresponding properties of foamsprepared in the manner described above from precursors containing higherand lower concentrations of 2,6-diaminopyridine.

Compression set of the foam was determined at 90 percent compressionaccording to ASTM Standard D-1564, Method B, using two steel plates heldparallel to each other by clamps. The space between the plates wasadjusted to the required thickness by spacers.

The results from the first stage of this study, which involved the useof 0.05 percent FSB surfactant in each precursor, are tabulated below inTable 1.

                                      TABLE 1                                     __________________________________________________________________________                            Density       % Loss After                                                                         Type                             Copolyimide                           30 Minutes                                                                           of                               System Composition                                                                              Mole Ratio                                                                          kg/m.sup.3                                                                        lbs/ft.sup.3                                                                      Resiliency                                                                          Recovery                                                                             Foam                             __________________________________________________________________________    1      BTDA:2,6DAP:MDA                                                                          1:0.4:0.6                                                                           14.7                                                                              0.92                                                                              85    21.0   Flexible, resilient,                                                          fine cellular                                                                 structure                        2      BTDA:2,6DAP:MDA                                                                          1:0.3:0.7                                                                           8.2 0.51                                                                              70    28.5   Flexible, resilient,                                                          fine cellular                                                                 structure                        3      BTDA:2,6DAP:MDA                                                                          1:0.2:0.8                                                                           6.7 0.42                                                                              60    41.3   Flexible, resilient,                                                          fine cellular                                                                 structure                        4      BTDA:2,6DAP:MDA                                                                          1:0.1:0.9                                                                           8.5 0.53                                                                              50    33.7   Flexible, resilient,                                                          fine cellular                                                                 structure                        __________________________________________________________________________     BTDA = 3,3',4,4benzophenonetetracarboxylic acid, methyl diester               2,6DAP = 2,6diaminopyridine                                                   MDA = p,pmetaphenylene diamine                                           

The data show that the copolyimide with the concentration ofheterocyclic diamine in the range critical to the success of ourinvention (system No. 1) had much better compression set properties thanthe other systems with which it is compared in Table 1.

The study was extended by decreasing the concentration of FSB surfactantin subsequently prepared precursors to 0.0125 percent and by increasingthe molar concentration of 2,6-diaminopyridine. The results thisproduced are shown below in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Copolyimide             Concentration                                                                         % Loss After                                  System                  of Surfactant                                                                         30 Minutes                                    Number Composition                                                                              Molar Ratio                                                                         (%)     Recovery                                                                             Type of Foam                           __________________________________________________________________________    2      BTDA:2,6DAP:MDA                                                                          1:0.3:0.7                                                                           0.0125  40.0   Fine, homogeneous cel-                                                        lular structure                        1      BTDA:2,6DAP:MDA                                                                          1:0.4:0.6                                                                           0.0125  19.6   Medium-large homogeneous                                                      cellular structure                     3      BTDA:2,6DAP:MDA                                                                          1:0.42:0.58                                                                         0.0125  12.1   Reticulated foam with                                                         medium size cellular                                                          structure                              4      BTDA:2,6DAP:MDA                                                                          1:0.44:0.56                                                                         0.0125  --     Highly reticulated foam                                                       with large and weak                                                           cellular structure                     5      BTDA:2,6DAP:MDA                                                                          1:0.5:0.5                                                                           0.0125  --     Highly reticulated foam                                                       with chopped strands                                                          like cell structure.                                                          Poor - hollow foam                     __________________________________________________________________________

The tabulated data again show that the foams made from precursors withconcentrations of heterocyclic diamines in the critical range had muchbetter compression set properties than those foams made from theprecursor with a lower concentration of the heterocyclic diamine.

At concentrations above the critical level, cell walls disappeared to anincreasing extent, being replaced by lamellar strands. This effect wasmost pronounced at the highest, 0.5 molar concentration of2,6-diaminopyridine. In that case the product was an unusable hollowshell with a thick, integral skin.

It was also found in the course of the study that the high degree ofreticulation obtained at a heterocyclic diamine molar concentration of0.44 is undesirable because it adversely effects fatigue resistance bycausing failure through a combination of cell fracture and shredding.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning andequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by U.S. Letters Patent is: 1.A polyimide foam precursor which is a resinoid obtained by combining atleast one alkyl ester of 3,3',4,4'-benzophenonetetracarboxylic acid; ameta- or para-substituted aromatic diamine which is free of aliphaticmoieties; and a heterocyclic diamine, said ester(s) and said diaminesbeing present in concentrations such that the imide formingfunctionalities are substantially equimolar, and the molar ratio ofheterocylic diamine to tetracarboxylic acid diester being in the rangeof about 0.4-0.42 to 1.0.
 2. A polyimide foam precursor as defined ineither of the preceding claims 1 or 2 which also contains from about0.00625 to about 0.05 weight percent of an effective surfactant.
 3. Apolyimide foam precursor as defined in either of the preceding claims 1or 2 in which the heterocyclic diamine is 2,6-diaminopyridine and thearomatic diamine is p,p'-methylene dianiline.